The present invention relates to magnetic metal particles containing iron as a main component, a process for producing the same and a magnetic recording medium using the same, and more particularly, to magnetic metal particles containing iron as a main component which have a high coercive force, a more excellent oxidation stability and a less soluble salt content in spite of fine particles having an average major axis diameter as small as 0.02 to 0.08 μm, a process for producing the magnetic metal particles, and a magnetic recording medium using the magnetic metal particles.
In recent years, miniaturization, lightening, recording-time prolongation and high-density recording as well as increase in memory capacity in magnetic recording and reproducing apparatuses for audio, video or computer have proceeded more rapidly. With such a recent tendency, it has been increasingly required to provide magnetic recording media having a high performance and a high-density recording property, such as magnetic tapes and magnetic disks.
Namely, the magnetic recording media have been required to have high image definition and quality, high output characteristics such as, in particular, good frequency characteristics, an excellent keeping property and a high durability. For this reason, it has been required that the magnetic recording media are reduced in noise due to the magnetic recording media themselves, and exhibit a high coercive force Hc, a narrow coercive force distribution (Switching Field Distribution: SFD) and an excellent weather resistance ΔBm.
These properties of the magnetic recording media have a close relation to magnetic particles used therein. In recent years, magnetic metal particles containing iron as a main component have been noticed because the magnetic metal particles can show a higher coercive force and a larger saturation magnetization σs as compared to those of conventional magnetic iron oxide particles, and have been already used as magnetic particles for magnetic recording media such as digital audio tapes (DAT), 8-mm video tapes, Hi-8 tapes, W-VHS tapes for Hi-Vision, DVC tapes of digital recording type, etc., as well as removable disks for computers such as Zip disks and Super disks. Further, it has been recently attempted to practically apply the magnetic metal particles containing iron as a main component to large-capacity Hi-FD.
Therefore, it has also been strongly required to improve properties of the magnetic metal particles containing iron as a main component.
More specifically, in order to obtain magnetic recording media satisfying various properties mentioned above, the magnetic metal particles containing iron as a main component which are used as magnetic particles therein, have been strongly required to be in the form of fine particles, and to exhibit a higher coercive force, an excellent oxidation stability Δσs and a lessened soluble salt content.
As to the reduction in particle size of the magnetic metal particles, in Japanese Patent Application Laid-Open (KOKAI) No. 2000-251243, it is described that “ . . . In order to achieve a high recording density of magnetic recording media, it has been intensively required to shorten the wavelength of signals to be recorded thereon. When the size of a magnetic material used becomes as large as that compatible with a length of a recording region for signals, a clear magnetization transition region is no longer available, so that it becomes substantially impossible to record signals thereon. For this reason, it has been longtime demanded to provide the magnetic material in the form of fine particles for achieving high-density recording upon use.”. Thus, in order to obtain magnetic recording media having a high output characteristics in a short wavelength region as well as a lessened noise, it is necessary to reduce the particle size of the magnetic metal particles, i.e., reduce the major axis diameter thereof to obtain fine particles.
Also, in recent years, it has been attempted to use a magneto resistance-type head as a reproduction head for computer tapes instead of conventional induction-type magnetic heads. The magneto resistance-type head can readily produce a considerably high reproduction output as compared to the conventional induction-type magnetic heads, and is free from impedance noise due to use of induction coil. Therefore, the use of the magneto resistance-type head contributes to reduction in a system noise to a large extent. If such noises due to magnetic recording media themselves are reduced, it will be possible to attain a high C/N ratio. Accordingly, in order to reduce such magnetic recording media noises, in particular, noises due to particles, it has been required to further reduce the particle size of the magnetic metal particles used therein.
On the other hand, the coercive force of the magnetic particles is generally produced due to shape anisotropy thereof. Therefore, in order to obtain magnetic particles having a high coercive force, it is required to increase an aspect ratio (average major axis diameter/average minor axis diameter) of the particles. However, since the aspect ratio of the magnetic particles tends to be lowered in proportion to reduction in particle size thereof, it may be difficult to obtain fine magnetic metal particles having a high coercive force. As to this face, in Japanese Patent Application Laid-Open (KOKAI) No. 10-83906(1998), it is described that “ . . . the coercive force of the above metal particles generally has a close relation to the size thereof. As the particles become finer, it is more difficult to maintain a high coercive force thereof. For example, if the minor axis diameter of acicular particles is kept constant, the coercive force thereof increases in proportion to the aspect ratio (major axis diameter/minor axis diameter) thereof. . . In order to achieve not only high coercive force but also high output in a short wavelength region, since the increase of major axis diameter of the particles is limited, only a way for increasing the aspect ratio and achieving a high coercive force is to shorten the minor axis diameter of the particles. However, it is known that when the minor axis diameter of the particles becomes too small, a so-called super paramagnetism occurs, thereby failing to show a coercive force. Thus, the reduction of minor axis diameter of the particles is also limited . . . ”.
It is also known that the reduction in particle size of the magnetic metal particles is accompanied with reduction of the minor axis diameter and crystallite size thereof and, therefore, increase of a specific surface area thereof, so that it is very difficult to maintain a high oxidation stability. Since the oxidation stability of the magnetic metal particles largely contributes to keeping property and durability of the resultant magnetic recording media, it has been strongly required to provide magnetic metal particles exhibiting a high oxidation stability in spite of fine particles.
In particular, with the recent tendency of reduction in noises of magnetic recording media, the magnetic metal particles used therein have been increasingly reduced in particle size. However, when the particle size of the magnetic metal particles is reduced to less than 0.05 μm, the surface activity thereof becomes remarkably high as compared to that of magnetic metal particles having a particle size of not less than 0.05 μm, so that the magnetic metal particles tend to be readily oxidized in air, resulting in significant deterioration in magnetic properties thereof with the passage of time. As a result, conventionally, as means for achieving a high C/N ratio by reducing noises due to magnetic recording media themselves, a person skilled in the art has been hardly motivated to use the magnetic metal particles having a particle size of less than 0.05 μm as magnetic particles for magnetic recording media.
On the other hand, the magnetic metal particles contain various impurities such as alkali metals due to production processes thereof.
Namely, the magnetic metal particles containing iron as a main component can be produced by using, as starting particles, (i) goethite particles obtained by passing an oxygen-containing gas such as air through a water suspension containing an iron-containing precipitate obtained by reacting an aqueous ferrous salt solution containing ferrous sulfate or the like with an aqueous alkali solution containing alkali salt such as sodium hydroxide and sodium carbonate, to conduct an oxidation reaction thereof, (ii) hematite particles obtained by heat-dehydrating the goethite particles, or (iii) particles obtained by incorporating different kinds of elements other than iron into these particles; and heat-reducing the starting particles under a reducing gas atmosphere.
Owing to the above production processes, the magnetic metal particles contain soluble sodium salt and sulfate ions as well as soluble calcium salt inevitably introduced during the production processes. When such magnetic metal particles containing the soluble sodium salt, the soluble calcium salt or the sulfate ions are used as magnetic particles for magnetic recording media, there arises such a problem that compounds derived from these soluble salts are precipitated on a magnetic coating film or a magnetic head. As to this fact, in Japanese Patent Application Laid-Open (KOKAI) No. 9-305958(1997), it is described that “when the total amount of water-soluble ions contained in magnetic materials, non-magnetic materials, carbon black and fillers used in the respective layers exceeded a certain amount, there was recognized such a phenomenon that the obtained magnetic tape suffered from increase in friction coefficient upon running the tape after stored under high-temperature and high-humidity conditions, and in extreme cases, the magnetic tape caused a “stuck” phenomenon and stopped. In still worse cases, solids precipitated on the magnetic tape cause a spacing loss, thereby deteriorating reproduction output of the magnetic tape. Further, the magnetic metal head is severely corroded, resulting in deteriorated recording and reproducing characteristics“.
Namely, in the case of the magnetic recording media produced by using fine magnetic metal particles not subjected to any treatment for reducing soluble salts thereof, for example, “magnetic recording media comprising a non-magnetic substrate; at least one undercoat formed on the non-magnetic substrate and composed of a binder and non-magnetic particles dispersed therein; and a magnetic layer formed as an uppermost layer on the undercoat and composed of a binder and ferromagnetic particles dispersed therein, which have an average major axis diameter of 0.01 to 0.06 μm, a crystallite size of 10 to 150 Å and an acicular ratio of 2:1 to 15:1, wherein the magnetic layer contains non-combustible components in an amount of 80 to 95%” as described in Japanese Patent Application Laid-Open No. 9-63040(1997), since the magnetic metal particles are not subjected to any treatment for reducing the soluble salts contained therein, compounds derived from the soluble salts contained in the magnetic metal particles tend to be eluted from the magnetic metal particles, then precipitated on the magnetic coating film, and adhered onto the magnetic head, resulting in spacing loss due to the precipitate. As a result, there may arise problems such as deteriorated reproduction output of magnetic tapes, corrosion of the metal head and deteriorated recording and reproducing characteristics.
In order to reduce amounts of the soluble salts contained in the magnetic metal particles, there may be used either 1) a method of producing the magnetic metal particles without using any aqueous alkali solution containing alkali metal salts such as sodium hydroxide, or 2) a method of removing the soluble salts from the particles by washing the particles with water.
As the method of removing the soluble salts from the particles by washing the particles with water, it may be considered that the intermediate product obtained in each stage of the production process of the magnetic metal particles is washed with water. However, in the production process of the magnetic metal particles, even though the goethite particles or hematite particles as starting particles at each stage are washed with water, only soluble salts present on the surface of the particles are removed. For this reason, even though such water-washed starting particles are subjected to reduction reaction to produce the magnetic metal particles, insoluble impurities contained within the particles are moved to the surface thereof, and are precipitated thereon in the form of corresponding soluble salts. Thus, it is impossible to completely remove these soluble salts from the magnetic metal particles. On the other hand, when the obtained magnetic metal particles are washed with water, the thus washed magnetic metal particles, especially spindle-shaped magnetic metal particles, tend to be deteriorated in magnetic properties such as coercive force as well as dispersibility in magnetic coating composition.
Thus, in the above-described conventional methods of removing the soluble salts by water-washing, although the soluble salt content is reduced to some extent, it may be difficult to completely remove the soluble salts from the particles, i.e., reduce the soluble salt content to zero. In addition, the conventional methods lead to deteriorated magnetic properties of the obtained particles. Under this circumstance, it has been required to minimize residual impurities contained in the particles by avoiding use of the aqueous alkali solution containing alkali metal salts such as sodium hydroxide, thereby producing high-purity magnetic metal particles.
Hitherto, there have been proposed methods for producing goethite particles without using any aqueous alkali solution containing alkali metals, or methods for water-washing hematite particles obtained by heat-dehydrating the goethite particles as well as the finally obtained magnetic metal particles (Japanese Patent Application Laid-Open (KOKAI) Nos. 61-174119(1986), 7-22224(1995), 7-326035(1995), 8-7256(1996), 8-185624(1996), 8-186015(1996), 8-279137(1996), 8-279138(1996), 8-306031(1996), 9-106535(1997), 9-305958(1997), 10-69629(1998), 10-83906(1998), 2001-81506 and 2001-176052, WO00/38201, Japanese Patent Application Laid-Open (KOKAI) No. 2001-192211, etc.).
At present, it has been strongly required to provide magnetic metal particles containing iron as a main component, which can show a high coercive force, an excellent oxidation stability and a less soluble salt content in spite of fine particles. However, the conventional methods have failed to obtain such magnetic metal particles containing iron as a main component which are capable of fully satisfying various properties mentioned above.
Namely, in Japanese Patent Application Laid-Open (KOKAI) No. 61-174119(1986), there is described the method of producing goethite particles by using an aqueous ammonium carbonate solution together with an aqueous ferrous sulfate solution. However, since the obtained goethite particles contain no cobalt, magnetic metal particles produced from such goethite particles as starting particles may fail to show a sufficient oxidation stability.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 7-22224(1995), it is described that hematite particles or magnetic metal particles are washed with water. However, the obtained magnetic metal particles have a major axis diameter of not less than 0.08 μm and a coercive force of not more than 2,000 Oe. Therefore, the obtained particles fail to fulfill requirements such as reduction in particle size and enhancement of coercive force.
In Japanese Patent Application Laid-Open (KOKAI) Nos. 7-326035(1995) and 8-7256(1996), there is described the method of producing magnetic metal particles from goethite particles obtained by using an aqueous ferrous salt solution, ammonium carbonate and aqueous ammonia. However, the obtained magnetic metal particles have an average major axis diameter as large as not less than 0.08 μm, and, therefore, also fail to fulfill requirements such as reduction in particle size and enhancement of coercive force.
In Japanese Patent Application Laid-Open (KOKAI) No. 8-185624(1996), it is described that hematite particles or magnetic metal particles are washed with water to control the ratio between sodium ions and potassium ions contained therein to a specific range. However, the obtained magnetic metal particles have an average major axis diameter as large as about 0.13 μm and a soluble sodium content as large as 200 ppm or lower, and, therefore, also fail to fulfill requirements such as reduction in particle size, enhancement of coercive force and improvement in oxidation stability.
Also, in Japanese Patent Application Laid-Open (KOKAI) Nos. 8-279137(1996) and 8-306031(1996), there is described the method of producing magnetic metal particles from goethite particles obtained by using an aqueous ferrous salt solution, ammonium carbonate and aqueous ammonia. The obtained magnetic metal particles have an average major axis diameter of 0.05 to 0.13 μm, and the surface layer portion thereof is higher in contents of aluminum, rare earth element and oxygen than those of whole particles. However, since an aluminum compound is added after completing the production reaction of the goethite particles and water-washing the goethite particles, no aluminum is contained within crystals of the goethite particles. Therefore, the reducing velocity upon the heat-reduction treatment is not sufficiently controlled, resulting in acceleration of sintering. As a result, the obtained particles fail to provide fine magnetic metal particles exhibiting excellent magnetic properties and oxidation stability.
In Japanese Patent Application Laid-Open (KOKAI) No. 9-106535(1998), there is described the method of producing magnetic metal particles from goethite particles obtained by using an aqueous ferrous salt solution, ammonium carbonate and aqueous ammonia. However, the obtained magnetic metal particles have a major axis diameter of 0.03 to 0.08 μm and a coercive force of 1,900 to 2,400 Oe, and also, fail to provide particles having an excellent oxidation stability.
In Japanese Patent Application Laid-Open (KOKAI) No. 9-305958(1997), it is described that the method of producing magnetic metal particles from goethite particles produced by using alkali carbonate containing no alkali metals, and further washing respective intermediate particles before producing the magnetic metal particles as final product with water in order to reduce a water-soluble ion content in the magnetic metal particles. However, since compounds containing alkali metals are used as additives, the obtained goethite particles still contain alkali metals and, therefore, fail to provide high purity goethite particles.
In Japanese Patent Application Laid-Open (KOKAI) No. 10-69629(1998), there is described the method of washing either goethite particles, hematite particles or magnetic metal particles with water. However, when the water-washed goethite particles or hematite particles are subjected to reduction reaction to produce the magnetic metal particles, insoluble impurities contained within these starting particles are moved onto the surface of the resultant magnetic metal particles, and precipitated thereon in the form of corresponding soluble salts. Thus, this method also fails to provide high purity goethite particles. On the other hand, when the magnetic metal particles are washed with water, the resultant particles tend to be deteriorated in magnetic properties such as saturation magnetization and coercive force as well as dispersibility in magnetic coating composition. Therefore, the resultant magnetic metal particles fail to show excellent magnetic properties.
In Japanese Patent Application Laid-Open (KOKAI) No. 10-83906(1998), there is described the method of producing goethite particles from ferrous chloride, alkali hydroxide composed of aqueous ammonia, and alkali carbonate such as ammonium carbonate. In this method, the particles are not washed with aqueous ammonia. Further, since the production reaction of the goethite particles is conducted at a high pH value, cobalt ions are eluted out in the form of an ammine complex. As a result, it may be difficult to obtain magnetic metal particles having a high coercive force.
In Japanese Patent Application Laid-Open (KOKAI) No. 9-171914(1997), there is described the method of producing magnetic metal particles by coating with a compound containing at least one element selected from the group consisting of Co, Al, Si and Ca together with a rare earth compound the surface of goethite particles; washing the goethite particles with water; coating with a carbon compound the surface of the goethite particles as an outermost layer; and then subjecting the thus treated goethite particles to heat-dehydration and reduction reaction. However, since cobalt and aluminum do not form a solid solution in the goethite particles, the obtained magnetic metal particles fail to show sufficient magnetic properties. In addition, although the goethite particles coated with the compound containing at least one element selected from the group consisting of Co, Al, Si and Ca and the rare earth compound is washed with water, when such goethite particles are subjected to reduction reaction to produce magnetic metal particles, insoluble impurities contained within the particles are moved to the surface thereof and precipitated thereon in the form of corresponding soluble salts. Therefore, the obtained goethite particles fail to provide high purity magnetic metal particles.
In Japanese Patent Application Laid-Open (KOKAI) No. 2001-81506, there is described the method of producing magnetic metal particles by coating with rare earth element and Si the surface of Co-containing iron oxide hydroxide or iron oxide particles, in which Al forms a solid solution, and then reducing the obtained particles with a reducing gas. However, the obtained magnetic metal particles have an average major axis diameter as large as 0.10 μm and, therefore, fail to fulfill the requirements such as reduction in particle size and enhancement of coercive force.
In Japanese Patent Application Laid-Open (KOKAI) Nos. 2001-176052 and 2001-176053, there is described the method of producing magnetic metal particles from goethite particles obtained by using an aqueous ferrous salt solution, ammonium carbonate and aqueous ammonia. However, since a cobalt compound is added only after completing the production reaction of goethite particles, it is not possible to obtain goethite particles in which cobalt is incorporated into goethite crystals. Therefore, in this method, it is considered that the reducing velocity upon the heat-reduction treatment is not sufficiently controlled, resulting in acceleration of sintering. As a result, the obtained magnetic metal particles fail to provide fine particles showing excellent magnetic properties and oxidation stability.
In WO00/38201, there is described the method of producing magnetic metal particles by mixing an aqueous ferric salt solution, an aqueous cobalt salt solution and water-soluble aluminum salt with an aqueous sodium hydroxide solution; aging the mixture to obtain iron oxide hydroxide particles containing cobalt and aluminum; coating the surface of the iron oxide hydroxide particles with a rare earth compound; and then heat-reducing the thus coated iron oxide hydroxide particles. However, when the iron oxide hydroxide particles obtained by using sodium hydroxide as an alkali source are subjected to reduction reaction to produce magnetic metal particles, insoluble impurities contained within the particles are moved onto the surface thereof, and precipitated thereon in the form of corresponding soluble salts. Therefore, the obtained particles fail to be sufficiently reduced in soluble salt content such as soluble Na salt content.
In Japanese Patent Application Laid-Open (KOKAI) No. 2001-192211, there is described the method of producing magnetic metal particles having an average major axis diameter of 0.05 to 0.2 μm from goethite particles obtained by using an aqueous ferrous salt solution, ammonium carbonate and aqueous ammonia. However, in this method, since the cobalt compound and rare earth compound are simultaneously coated on the surface of the goethite particles, a sufficient anti-sintering effect cannot be attained upon the reduction treatment. Therefore, it may be difficult to obtain magnetic metal particles in the form of fine particles which have an average major axis diameter of from 0.02 to less than 0.05 μm, a high coercive force and an excellent oxidation stability.
Therefore, it has been required to provide magnetic metal particles in the form of fine particles having an average major axis diameter as small as 0.02 to 0.05 μm and exhibiting an excellent oxidation stability as well as such a high purity that the content of residual impurities is close to zero.
As a result of the present inventors' earnest studies for solving the above problems, it has been found that by reacting an aqueous ferrous sulfate solution with a mixed aqueous alkali solution composed of an aqueous ammonium hydrogen carbonate solution in an amount of 1.7 to 3.0 equivalents based on equivalent of the aqueous ferrous sulfate solution and an aqueous ammonium hydroxide solution in an amount of 55 to 85 mol % based on the mixed aqueous alkali solution, to obtain a water suspension containing a ferrous-containing precipitate;
aging the water suspension containing the ferrous-containing precipitate in a non-oxidative atmosphere;
adding a Co compound in an amount of 10 to 35 atm % (calculated as Co) based on whole Fe, to the water suspension containing the ferrous-containing precipitate during the aging;
passing an oxygen-containing gas through the water suspension containing the ferrous-containing precipitate to conduct an oxidation reaction thereof until 20 to 80% of whole Fe2+ is oxidized, to produce goethite seed crystal particles;
upon growing a goethite layer on the surface of the goethite seed crystal particles by passing an oxygen-containing gas through the water suspension containing the goethite seed crystal particles and the ferrous-containing precipitate, adding an Al compound in an amount of 3 to 15 atm % (calculated as Al) based on whole Fe, to the water suspension containing the goethite seed crystal particles and the ferrous-containing precipitate so as to adjust a pH value of the water suspension to less than 9.0, thereby producing goethite particles;
after filtering out the thus produced goethite particles from the water suspension, washing the goethite particles with aqueous ammonia having a pH value of 9.5 to 11.5 to obtain goethite particles;
adding a cobalt compound and alkali carbonate to a water dispersion containing the goethite particles to form a cobalt carbonate coat in an amount of 10 to 25 atm % (calculated as Co) based on whole Fe on the surface of the goethite particles;
adding a rare earth compound to the water dispersion to form a rare earth compound coat in an amount of 3 to 20 atm % (calculated as rare earth element) based on whole Fe on the surface of the cobalt carbonate coat; and
heat-reducing either the surface-coated goethite particles or hematite particles obtained by heat-treating the surface-coated goethite particles at a temperature of 400 to 750° C. in a non-reducing atmosphere, at a temperature of 350 to 700° C. in a reducing atmosphere,
the thus obtained magnetic metal particles containing iron as a main component can exhibit a high coercive force, an excellent oxidation stability and a less soluble salt content in spite of fine particles. The present invention has been attained on the basis of this finding.